Doped scandia stabilized zirconia electrolyte compositions

ABSTRACT

A solid oxide fuel cell (SOFC) electrolyte composition includes zirconia stabilized with scandia, and at least one of magnesia, zinc oxide, indium oxide, and gallium oxide, and optionally ceria in addition to the oxides above.

FIELD

The present invention is generally directed to fuel cell components, andto solid oxide fuel cell electrolyte materials in particular.

BACKGROUND OF THE INVENTION

Fuel cells are electrochemical devices which can convert chemical energystored in fuels to electrical energy with high efficiencies. Theycomprise an electrolyte between electrodes. Solid oxide fuel cells(SOFCs) are characterized by the use of a solid oxide as theelectrolyte.

In solid oxide fuel cell (SOFC) system, an oxidizing flow is passedthrough the cathode side of the fuel cell while a fuel flow is passedthrough the anode side of the fuel cell. The oxidizing flow is typicallyair, while the fuel flow can be a hydrocarbon fuel, such as methane,natural gas, pentane, ethanol, or methanol. The fuel cell, operating ata typical temperature between 650° C. and 950° C., enables the transportof negatively charged oxygen ions from the cathode flow stream to theanode flow stream, where the ion combines with either free hydrogen orhydrogen in a hydrocarbon molecule to form water vapor and/or withcarbon monoxide to form carbon dioxide. The excess electrons from thenegatively charged ion are routed back to the cathode side of the fuelcell through an electrical circuit completed between anode and cathode,resulting in an electrical current flow through the circuit.

In recent years considerable interest has been shown towards thedevelopment of SOFC electrolyte compositions of high ionic conductivity.Doped cerium oxide, lanthanum gallate and zirconium oxide are the mostsuitable candidates. However, in order to achieve sufficient ionicconductivity high operation temperature is often required, deterioratingthe life of the fuel cell components and requiring the use of expensivematerials in the fuel cell stack, such as chromium alloy interconnects.Therefore, it is highly desirable to lower the operation temperature ofSOFCs and one important step to achieve this is the development of anelectrolyte composition with higher ionic conductivity thanyttria-stabilized zirconia (YSZ), the state of the art SOFC electrolytematerial.

Doping zirconia with aliovalent dopants stabilizes the high temperaturecubic fluorite phase at room temperature leading to an increase inoxygen vacancy concentration, oxygen mobility and ionic conductivity.Complex studies have demonstrated a correlation between the dopant andhost ionic radii and the existence of a critical dopant cation radiusthat can ensure maximum conductivity. It has been suggested that a goodevaluation of the relative ion mismatch between dopant and host would beto compare the cubic lattice parameter of the host oxide and thepseudocubic lattice parameter of the dopant oxide, a smaller sizemismatch being preferred for obtaining high ionic conductivity.

Numerous attempts to find the appropriate dopant for stabilizing thecubic phase have been made (Y, Yb, Ce, Bi, etc). Among these, scandiastabilized zirconia shows the highest ionic conductivity with Sc³⁺concentration of 11 mole % (2-3 higher than YSZ at 800° C.) due to alower activation energy than YSZ. The complex phase diagram in theSc₂O₃—ZrO₂ system is still under debate, with several phases identifiedin the dopant rich segment of the phase diagram, as monoclinic,tetragonal and rhombohedral intermediate phases appear at lowtemperatures. One example is the distorted rhombohedral β phase(Sc₂Zr₇O₁₇), that undergoes a rhombohedral-cubic phase transition around600-700° C. and induces a steep decrease in conductivity in thistemperature region. This transition is not favorable from the point ofview of thermal expansion mismatch and may be related to order-disordertransition of oxygen vacancies.

Co-doping in zirconia systems may produce cheaper, stable compositionswith enhanced ionic conductivity. Co-dopants in the scandia zirconiasystem include Ce, Y, Yb and Ti, the former (1 mole % CeO₂) being themost successful to date in stabilizing the cubic phase at roomtemperature and very high conductivity values have been measured by Leeet al, 135 mS/cm at 800° C. in air (SSI 176 (1-2) 33-3 (2005)). However,concerns about long term stability, especially at the interface with thefuel electrode, due to Ce³⁺ presence, have been reported.

In general, CaO or MgO are used for improving the toughness of zirconiaceramics by stabilizing the tetragonal phase at room temperature.

However, previous studies for doping zirconia with alkaline earth metalcations alone have been unsuccessful in improving the conductivitybecause of a high tendency of defect association and a lowerthermodynamic stability of cubic fluorite ZrO₂—CaO and ZrO₂—MgO solidsolutions.

SUMMARY OF THE INVENTION

A solid oxide fuel cell (SOFC) electrolyte composition includes zirconiastabilized with scandia, and at least one of magnesia, zinc oxide,indium oxide, and gallium oxide, and optionally ceria.

Another embodiment of the invention provides an electrolyte compositionfor a solid oxide fuel cell that includes zirconia stabilized withscandia and indium oxide, in which scandia and indium oxide are presentin total amount that is greater than or equal to 10 mol % and less thanor equal to 13 mol %. Another embodiment of the invention provideszirconia stabilized with scandia, indium oxide, and ceria, in whichscandia, indium oxide, and ceria are present in a total amount that isgreater than or equal to 8 mol % and less than or equal to 14 mol %,such as 11 mol %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D are ternary phase diagrams illustrating embodiment series ofmagnesia doped scandia stabilized zirconia compositions.

FIGS. 2A-2D are plots showing x-ray diffraction patterns forcompositions in embodiment series of magnesia doped scandia stabilizedzirconia compositions.

FIG. 3 is a ternary phase diagram illustrating the structures ofcompositions in embodiment series of magnesia doped scandia stabilizedzirconia compositions.

FIGS. 4A and 4B are back scattered electron images from samplecompositions in embodiment series of magnesia doped scandia stabilizedzirconia compositions.

FIG. 5A is a graph showing D.C. conductivity versus atomic percent ofmagnesium ions at 850° C. for embodiment series of magnesia dopedscandia stabilized zirconia compositions.

FIGS. 5B-5G are graphs showing electrical impedance spectroscopy resultsfor sample compositions in embodiment series of magnesia doped scandiastabilized zirconia compositions.

FIG. 6A is a graph showing D.C. conductivity versus atomic percent ofscandium ions at 850° C. for embodiment series of magnesia doped scandiastabilized zirconia compositions.

FIG. 6B is a graph showing D.C. conductivity versus oxygen stoichiometryat 850° C. for embodiment series of magnesia doped scandia stabilizedzirconia compositions.

FIG. 6C is a ternary phase diagram showing D.C. conductivity results at850° C. for embodiment series of magnesia doped scandia stabilizedzirconia compositions.

FIG. 7A is a ternary phase diagram of two embodiment series of yttriaand magnesia doped scandia stabilized zirconia compositions.

FIGS. 7B and 7C are plots showing x-ray diffraction patterns forcompositions in two embodiment series of yttria and magnesia dopedscandia stabilized zirconia compositions.

FIG. 8A is a graph showing D.C. conductivity versus atomic percentage ofyttrium ions at 850° C. for two embodiment series of yttria and magnesiadoped scandia stabilized zirconia compositions.

FIGS. 8B-8E are graphs showing electrical impedance spectroscopy resultsfor sample compositions in an embodiment series of yttria and magnesiadoped scandia stabilized zirconia compositions.

FIGS. 9A-9D are graphs showing electrical impedance spectroscopy resultsfor sample compositions in two embodiment series of yttria and magnesiadoped scandia stabilized zirconia compositions.

FIG. 10A is a graph showing D.C. conductivity versus atomic percentageof scandium ions at 850° C. for embodiment series of magnesia dopedscandia stabilized zirconia compositions and yttria and magnesia dopedscandia stabilized zirconia compositions.

FIG. 10B is a graph showing D.C. conductivity versus oxygenstoichiometry at 850° C. for embodiment series of magnesia doped scandiastabilized zirconia compositions and yttria and magnesia doped scandiastabilized zirconia compositions.

FIG. 11A is a plot showing x-ray diffraction patterns for samplecompositions in an embodiment series of zinc oxide doped scandiastabilized zirconia compositions.

FIG. 11B is an expanded view of the x-ray diffraction patterns of FIG.11A for angles within the range of 2θ=25-50.

FIG. 11C is an expanded view of the x-ray diffraction patterns of FIG.11A for angles within the range of 2θ=80-88.

FIG. 12A is a graph comparing D.C. conductivities at 850° C. of anembodiment series of magnesia doped scandia stabilized zirconiacompositions and a similar series of zinc oxide doped scandia stabilizedzirconia compositions.

FIG. 12B is a plot showing x-ray diffraction patterns for an embodimentseries of indium oxide doped scandia stabilized zirconia compositions.

FIG. 12C is a graph showing D.C. conductivity versus mole percent ofindium oxide at 850° C. for an embodiment series of indium oxide dopedscandia stabilized zirconia compositions.

FIG. 13A is plot showing electron diffraction spectroscopy patterns foran embodiment series of indium oxide and magnesia doped scandiastabilized zirconia compositions and an embodiment series of indiumoxide doped scandia stabilized zirconia compositions.

FIG. 13B is a plot showing x-ray diffraction patterns for samplecompositions in embodiment series of scandia stabilized zirconiacompositions doped with varying amounts of indium oxide and magnesia.

FIGS. 14A-14D are graphs showing electrical impedance spectroscopyresults for sample compositions in embodiment series of scandiastabilized zirconia compositions doped with varying amounts of yttria,magnesia, indium oxide, and/or zinc oxide.

FIG. 15 is a ternary phase diagram of embodiment series of scandiastabilized zirconia compositions doped with varying amounts of yttriaand gallium oxide.

FIG. 16A is a plot showing x-ray diffraction patterns for samplecompositions in embodiment series of scandia stabilized zirconiacompositions doped with varying amounts of yttria and gallium oxide.

FIG. 16B is an expanded view of the x-ray diffraction patterns of FIG.16A for angles within the range of 2θ=82-85.

FIG. 17A is a graph showing D.C. conductivity versus atomic percentageof yttrium and gallium ions at 850° C. for embodiment series of scandiastabilized zirconia compositions doped with varying amounts of yttriaand gallium oxide.

FIG. 17B is a graph showing D.C. conductivity versus atomic percentageof scandium ions at 850° C. for embodiment series of scandia stabilizedzirconia compositions doped with varying amounts of yttria and galliumoxide.

FIG. 18 is a graph showing D.C. conductivity versus atomic percentage ofgallium ions at 850° C. for an embodiment series of scandia stabilizedzirconia compositions doped with yttria, gallium oxide, and magnesia.

FIG. 19A is a graph showing D.C. conductivities as a function ofscandium content for sample compositions of various series of scandiastabilized zirconia doped with indium oxide and ceria at 850° C.

FIG. 19B is a graph showing D.C. conductivities as a function of indiumcontent for sample compositions of various series of scandia stabilizedzirconia doped with indium oxide and ceria at 850° C.

FIG. 19C is a graph showing D.C. conductivities as a function of theratio of scandium to indium for sample compositions of various series ofscandia stabilized zirconia doped with indium oxide and ceria at 850° C.

FIG. 20A is a graph showing D.C. conductivity as a function of scandiumcontent at 850° C. for embodiment series of scandia stabilized zirconiacompositions with various dopant combinations.

FIG. 20B is a graph showing D.C. conductivity as a function of oxygenstoichiometry at 850° C. for embodiment series of scandia stabilizedzirconia compositions with various dopant combinations.

FIG. 21 is a ternary phase diagram showing sample high conductivitycompositions of two embodiment series of scandia stabilized zirconiacompositions doped with magnesia.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The various embodiments provide compositions of an electrolyte for aSOFC which includes a doped scandia stabilized zirconia. In anembodiment, zirconia is co-doped with scandium and aliovalent atoms andis made by co-precipitation.

As both phase composition and conductivity may be very much dependant onthe synthesis conditions, numerous studies have focused on developingtechnologies for synthesis and sintering. Solid state synthesis is knownto lead to phase inhomogeneity due to the slow kinetics for cationmigration, therefore high sintering temperatures may be required forphase formation. Alternative techniques such as co-precipitation,combustion and sol-gel may prove to be more successful in achievingcompositional homogeneity and high extent of densification.

In the various embodiments, powder may be obtained usingco-precipitation, which includes dissolving stoichiometric amounts ofscandium, and at least one of magnesium oxide or carbonate, zinc oxide,indium oxide and/or gallium oxide (and optionally yttrium oxide inaddition to the above oxides depending on the composition) in hot HNO₃followed by mixing with and aqueous solution in which zirconiumacetylacetonate or other Zr precursor compound has been dissolved. Themixture may be stirred under heating on a hot plate then cooled down toroom temperature and precipitated with ammonia until pH=9. The formedprecipitate may be filtrated, dried and calcined at 1200° C. for 5hours. The resulting powder may be crushed, ball milled and pressed intopellets and bars to be sintered to dense bodies at 1500-1550° C. for 7hours. The sintered product may be characterised using X-raydiffraction, particle size analysis, SEM, TEM and conductivitymeasurements. To obtain an accurate value of the ionic conductivity athigh temperatures, the bulk and grain boundary contributions to thetotal resistance of the sample may be separated out.

In preparing these new electrolyte compositions, a parent electrolytematerial may have a molar ratio of zirconia (ZrO₂):scandia (Sc₂O₃) thatis around 89:11, such as 87-91:13-9. In an embodiment, zirconia may bedoped with magnesia (MgO), or other ionic oxide with an aliovalentcation (e.g., Mg²⁺), up to 11% mole percent (mol %), while keeping theatomic percent of either Sc or Zr constant. In other embodiments,zirconia may be doped with a combination of one or more of magnesia,yttria, zinc oxide, and/or indium oxide. In other embodiments, zirconiamay be doped with a combination of one or more of magnesia, yttria, andgallium oxide.

In an embodiment, magnesia may be used as a dopant that replaces scandiain scandia stabilized zirconia. Four example series of compositions arediscussed in further detail below.

One example series of compositions (“A-series”) may be prepared based onthe composition of 11 mol % Sc₂O₃, and may have a formula ofZr_(0.802)Sc_(0.198-x)Mg_(x)O_(1.90-0.5x). A ternary phase diagramshowing the compositions of this example A-series is illustrated in FIG.1A. In the A-series compositions, Sc³⁺ ions may be replaced by Mg²⁺ ionsin a 1:1 ratio, thereby lowering scandium and oxygen content of thecomposition, while keeping zirconia content constant. The x values inthe above formula that may be used to form the A-series are: 0, 0.009,0.018, 0.027 and 0.036, thereby creating the following compositions:

-   A0: Zr_(0.802)Sc_(0.198)O_(1.90)-   A1: Zr_(0.802)Sc_(0.189)Mg_(0.009)O_(1.90)-   A2: Zr_(0.802)Sc_(0.180)Mg_(0.018)O_(1.89)-   A3: Zr_(0.802)Sc_(0.171)Mg_(0.027)O_(1.89)-   A4: Zr_(0.802)Sc_(0.162)Mg_(0.036)O_(1.88)

In this example series, at x=0, no Sc³⁺ ions are replaced, and thereforethe atomic percent of Sc³⁺ ions is equal to the atomic percent in theparent material (i.e., 19.8%). At the highest x value tested (x=0.36),the atomic percent of scandium ions becomes the lowest (i.e., 16.2%).

Another example series of compositions (“B-series”) may be preparedbased on the composition of 11 mol % Sc₂O₃. A formula for the B-seriescompositions may be Zr_(0.802+x)Sc_(0.198-2x)Mg_(x)O_(1.90) FIG. 1B is aternary phase diagram showing the compositions of the B-series. In thisexample series, two Sc³⁺ ions may be replaced by one Zr⁴⁺ and one Mg²⁺ion, thereby lowering scandium content while keeping the oxygen contentand stoichiometry constant at 1.90. The x values that may be used toform this B-series are: 0, 0.009, 0.0135, 0.018 0.027 and 0.036, therebycreating the following compositions:

-   B0: Zr_(0.802)Sc_(0.198)O_(1.90)-   B1: Zr_(0.811)Sc_(0.18)Mg_(0.009)O_(1.90)-   B1.5: Zr_(0.815)Sc_(0.171)Mg_(0.0135)O_(1.90)-   B2: Zr_(0.820)Sc_(0.162)Mg_(0.018)O_(1.90)-   B3: Zr_(0.829)Sc_(0.144)Mg_(0.027)O_(1.90)-   B4: Zr_(0.838)Sc_(0.126)Mg_(0.036)O_(1.90)

In this example series, at x=0, no Sc³⁺ ions are replaced, and thereforethe atomic percent of Sc³⁺ ions is equal to the atomic percent in theparent material (i.e., 19.8%). At the highest x value tested (x=0.036),the atomic percent of scandium ions is the lowest (i.e., 12.6%). Aternary phase diagram showing the compositions of this example B-seriesis illustrated in FIG. 1B.

Other example series of compositions (“G-series” and “H-series”) maymaintain constant scandia content and increase magnesia levels byreplacing Zr ions with Mg ions, thereby lowering the levels of zirconiumand oxygen. The example G-series of compositions may be prepared basedon a parent composition with 5.3 mol % Sc₂O₃, and may have a formula ofZr_(0.9-x)Sc_(0.1)Mg_(x)O_(1.95-x). A ternary phase diagram showing thecompositions in this example G-series is illustrated in FIG. 1C. The xvalues that may be used to form the G-series compositions are: 0, 0.025,0.05, 0.075 and 0.10, thereby creating the following compositions:

-   G0: Zr_(0.9)Sc_(0.1)O_(1.95)-   G1: Zr_(0.875)Sc_(0.1)Mg_(0.025)O_(1.925)-   G2: Zr_(0.85)Sc_(0.1)Mg_(0.05)O_(1.90)-   G3: Zr_(0.825)Sc_(0.1)Mg_(0.075)O_(1.875)-   G4: Zr_(0.80)Sc_(0.1)Mg_(0.10)O_(1.85)

At x=0, no Mg²⁺ ions are added, and therefore the atomic percent of Sc³⁺ions is equal to the atomic percent in the parent material (i.e.,10.0%). At the highest x value (x=0.10), the atomic percent of zirconiumions is the lowest (i.e., 80.0%).

Another example series of compositions (“H-series”) may be preparedbased on a parent composition of 8.1 mol % Sc₂O₃. The H-seriescompositions may have a formula Zr_(0.85-x)Sc_(0.15)Mg_(x)O_(1.925-x). Aternary phase diagram showing the example H-series compositions isillustrated in FIG. 1D. The x values that may be used to form theH-series compositions are: 0, 0.025, 0.05, 0.075 and 0.10, therebycreating the following compositions:

-   H0: Zr_(0.85)Sc_(0.15)O_(1.925)-   H1: Zr_(0.825)Sc_(0.15)Mg_(0.025)O_(1.90)-   H2: Zr_(0.80)Sc_(0.15)Mg_(0.05)O_(1.875)-   H3: Zr_(0.775)Sc_(0.15)Mg_(0.075)O_(1.85)-   H4: Zr_(0.75)Sc_(0.15)Mg_(0.10)O_(1.825)

X-ray diffraction patterns may determine the stable phase at roomtemperature for each magnesia doped scandia stabilized zirconiacomposition. The phases at room temperature for compositions in theexample A-, B-, G-, and H-series of compositions are shown in Table 1below:

Composition Phases Present Spacegroup A0, B0 Rhombohedral R-3C A1Rhombohedral R-3C A2 Cubic Fm-3m A3 Cubic Fm-3m A4 Cubic Fm-3m B1Tetragonal P42 nmc B1-5 Tetragonal P42 nmc B2 Tetragonal P42 nmc B3Tetragonal P42 nmc B4 Tetragonal P42 nmc G0 Tetragonal + monoclinic P42nmc+ G1 Tetragonal + monoclinic P42 nmc+ G2 Tetragonal + monoclinic P42nmc+ G3 Tetragonal + monoclinic P42 nmc+ G4 Tetragonal + monoclinic P42nmc+ H0 Tetragonal P42 nmc H1 Tetragonal P42 nmc H2 Cubic Fm-3m H3 CubicFm-3m H4 Tetragonal P42 nmcPrevious studies of the parent composition A0, B0 of the A and B-series,11 mol % Sc₂O₃, have found it to have a rhombohedral structure at roomtemperature. At the lowest level of doping in the A-series ofcompositions (x=0.09), the structure may remain rhombohedral, while atall other doping levels the cubic fluorite structure may be stable. Forthe example B-series, all compositions may have a tetragonal fluoritestructure. FIG. 2A illustrates an x-ray diffraction patterns for theexample B-Series compositions.

X-ray analysis of the magnesium free G0 sample showed both tetragonaland monoclinic fluorite phases to be present, consistent with findingsby Ruh et al. (Ruh 1977), in their study. FIG. 2B illustrates x-raydiffraction (XRD) patterns for G-series compositions, which show thatincreasing the magnesia content across the G-Series leads to a reductionin the amount of monoclinic phase present. For example, FIG. 2C, whichis a close up view of the low angle region in FIG. 2B illustrates x-raydiffraction patterns for G-series compositions, which show relative peakheights of the monoclinic reflections reduce with increasing magnesiacontent.

With respect to the H-series of compositions, additions of MgO withbetween 2.5 and 7.5 at. % magnesium stabilize the cubic phase. When themagnesium content is increased to 2.5 at. % the structure may remaintetragonal, but when the magnesium content is further increased to 5.0and 7.5 at. %, the cubic structure may be stabilized. A further increasein magnesium content to 10 at. % tetragonal structure may become stable.This is shown in FIG. 2D which is an XRD pattern series for the H-seriescompositions. A ternary phase diagram showing as fired phases for theB-, G- and H-series compositions is illustrated in FIG. 3.

FIGS. 4A and 4B are back scattered electron (BSE) images from samples B2and H2 respectively, taken using a scanning electron microscope (SEM).As shown by the data, sample G2 may have a much finer grain structure,which is consistent with the presence of tetragonal and monocliniczirconia. In contrast sample H2 may have a much coarser microstructure,typical of cubic zirconia.

FIG. 5A illustrates the variation of example D.C. conductivitymeasurements with magnesia content (measured atomic percent ofmagnesium) at 850° C. for the B, G and H-Series of compositions. Peakconductivity may be measured at or above 200 mS/cm, such as 200-210mS/cm for the B1.5 composition(Zr_(0.815)Sc_(0.171)Mg_(0.0135)O_(1.90)).

Regarding the G-series of compositions, conductivity may remain lessthan 100 m/Scm across the entire range of compositions. These relativelylow conductivities may be consistent with XRD results that show thepresence of unwanted low conductivity monoclinic phases at roomtemperature. These low results may also indicate that G-series levels ofmagnesia and scandia are likely insufficient to stabilize the moreconductive cubic phase of zirconia at 850° C. In contrast, the exampleH-series of compositions may show conductivities above 150 mS/cm, suchas 150-199 mS/cm at compositions with 5.0 at. % magnesium or less. Theconductivity may increase approximately linearly with magnesia contentto a peak value of 199 mS/cm for the H2 sample, (5.0 at. % magnesium). Afurther increase to 7.5 at. % magnesium may lead to a large drop inconductivity to 79 mS/cm.

FIG. 5B illustrates electrical impedance spectroscopy (EIS) measurementsshowing bulk conductivity for the sample G2 and H2 compositions with 5at. % magnesium. FIG. 5C illustrates EIS measurements showingconductivity across the grain boundary of the sample G2 and H2compositions. At 400° C. the low scandia G2 sample may have a higherbulk conductivity but lower grain boundary conductivity than the higherscandia H2 sample. As temperature increases the bulk and grain boundarycomponent of the conductivity may increase faster for the higher scandiasample, leading to the far superior conductivity at 850° C. Withoutwishing to be bound by a particular theory, the large grain boundaryresistance of the G-series phase may be the result of the presence ofthe lower conductivity monoclinic phase.

When the magnesium content of the H-series of compositions is increasedfrom 5 at. % to 7.5 at. % or higher, a large decrease in conductivitymay be observed. FIG. 5D illustrates EIS measurements showing totalconductivity for H2 and H3 samples, which have 5 at. % and 7.5 at. %magnesium, respectively. FIG. 5E illustrates EIS measurements showingconductivity across the grain boundary for H2 and H3. FIG. 5Fillustrates EIS measurements showing bulk conductivity for H2 and H3samples, while FIG. 5G illustrates bulk resistivity measurements for theH2 and H3 samples. It may be observed from these measurements that H2and H3 samples may have very similar bulk conductivities, while H3 mayhave a much lower grain boundary conductivity. Thus, 1-6 atomic percentMgO, such as 1.3-5 atomic percent are preferred.

FIG. 6A illustrates the variation of D.C. conductivity measurements withscandia content (measured as atomic percent of scandia) at 850° C. forthe B, G and H-Series of compositions. FIG. 6B illustrates the variationof D.C. conductivity measurements with oxygen stoichiometry at 850° C.for the B, G and H-Series of compositions. FIG. 6C illustrates the D.C.conductivity results at 850° C. are presented on a ternary compositiondiagram. These plots indicate that peak values of conductivity may beachieved at scandia contents between 15 and 19 at. %, magnesia contentsless than 5 at. % (e.g., 2-5 at. %), and oxygen stoichiometries between1.875 and 1.9.

In order to further decrease the level of scandia, two additional seriesof compositions based on the B1.5 and B3 compositions may be developed.

In another embodiment, scandia stabilized zirconia compositions may beco-doped with magnesia and yttria. An example series of compositions(“E-series”) may be prepared based on a parent composition of 10.7 mol %Sc₂O₃. The E-series may have a formulaZr_(0.815)Sc_(0.171-x)Y_(x)Mg_(0.0135)O_(1.90). In this series ofcompositions, one Y³⁺ ion replaces one scandium ion, while zirconium,magnesium and oxygen levels remain constant. The x values that may beused to form the E-series compositions were: 0, 0.018, 0.036, 0.054, and0.072, thereby creating the following E-series compositions:

-   E0: Zr_(0.815)Sc_(0.171)Mg_(0.0135)O_(1.90). (Same as B1.5)-   E1: Zr_(0.815)Sc_(0.153)Y_(0.018)Mg_(0.0135)O_(1.90)-   E2: Zr_(0.815)Sc_(0.135)Y_(0.036)Mg_(0.0135)O_(1.90)-   E3: Zr_(0.815)Sc_(0.117)Y_(0.054)Mg_(0.0135)O_(1.90)-   E4: Zr_(0.815)Sc_(0.099)Y_(0.072)Mg_(0.0135)O_(1.90)    At x=0, no scandium ions are replaced, and the atomic percent of    scandium is equal to the parent composition (i.e., 17.1%). At the    highest x value, x=0.072, atomic percent of scandium is lowest of    the series (i.e., 9.9%).

Another example series of compositions (“F-series”) may be preparedbased on a parent composition of 7.9 mol % Sc₂O₃. The F-series may havea formula Zr_(0.829)Sc_(0.144-x)Y_(x)Mg_(0.027)O_(1.90). The x valuesthat may be used to prepare the F-series compositions are: 0, 0.018,0.036, 0.054, and 0.072, thereby creating the following compositions:

-   F0: Zr_(0.829)Sc_(0.144)Mg_(0.027)O_(1.90) (Same as B3)-   F1: Zr_(0.829)Sc_(0.126)Y_(0.018)Mg_(0.027)O_(1.90)-   F2: Zr_(0.829)Sc_(0.108)Y_(0.036)Mg_(0.027)O_(1.90)-   F3: Zr_(0.829)Sc_(0.09)Y_(0.054)Mg_(0.027)O_(1.90)-   F4: Zr_(0.829)Sc_(0.072)Y_(0.072)Mg_(0.027)O_(1.90)

In this series of compositions, like in the E-series, one Y³⁺ ionreplaces one scandium ion, while zirconium, magnesium and oxygen levelsremain constant. At x=0, no scandium ions are replaced, and the atomicpercent of scandium is equal to the parent composition (i.e., 14.4%). Atthe highest x value, x=0.072, atomic percent of scandium is lowest ofthe series (i.e., 7.2%). A ternary phase diagram of the example E- andF-series compositions is illustrated in FIG. 7A (yttria not shown).

The XRD patterns of the example in the E-series and F-seriescompositions are shown in FIGS. 7B and 7C, respectively. The E-series ofcompositions may be tetragonal at yttrium contents of 5.4 at. % or lessand may be cubic at 7.2 at. %. The F-series compositions may remaintetragonal. At 5.4 at. % and 7.2 at. % yttrium, the F-series samples mayalso have small amounts of monoclinic phase. Table 2 shows the roomtemperature phases of example compositions E0 through E4, and F0 throughF4.

Composition Phases Present Spacegroup E0 (B1-5) Tetragonal P42 nmc E1Tetragonal P42 nmc E2 Tetragonal P42 nmc E3 Tetragonal P42 nmc E4 CubicFm3m F0 Tetragonal P42 nmc F1 Tetragonal F2 Tetragonal P42 nmc F3Tetragonal + monoclinic P42 nmc F4 Tetragonal + monoclinic P42 nmc

FIG. 8A illustrates the variation in D.C. conductivity with yttriacontent measured for the E and F series of compositions at 850° C. Asthe data show, an approximately linear decrease in conductivity mayoccur with increasing content. The highest conductivity for ayttria-containing sample may be 199 mS/cm for E1, which has 1.8 at. %yttrium.

FIG. 8B illustrates EIS measurements showing total conductivity for theE-series compositions E1, E3 and E4. FIG. 8C illustrates EISmeasurements showing bulk conductivity for E1, E3 and E4. FIG. 8Dillustrates EIS measurements showing conductivity across the grainboundary plane for E1, E3 and E4. FIG. 8E illustrates bulk resistivityvalues at 400° C. for E1, E3 and E4.

FIG. 9A illustrates EIS measurements showing total conductivity for thesamples E1 and F1, each of which have 1.8 at. % yttrium. FIG. 9Billustrates EIS measurements showing bulk conductivity for E1 and F1.FIG. 9C illustrates EIS measurements showing conductivity across thegrain boundary plane for E and F1. FIG. 9D illustrates bulk resistivityvalues at 400° C. for E1 and F1.

FIG. 10A illustrates D.C. conductivity at 850° C. versus scandia contentfor series A, B, G and H (ZrO₂—Sc₂O₃—MgO), and E and F(ZrO₂—Sc₂O₃—Y₂O₃—MgO) at 850° C. Further, FIG. 10 B illustrates D.C.conductivity at 850° C. versus oxygen stoichiometry for each of theseseries. The conditions for high conductivity in the yttria containingsamples are consistent with those found for the yttria free samples. Thehighest values of conductivity may be found for scandium contentsbetween 15 and 19 at. % magnesium contents less than 5 at. % and oxygenstoichiometries between 1.875 and 1.9.

In another embodiment, scandia stabilized zirconia compositions that aresimilar to the B-series compositions may be doped with either zinc oxide(ZnO) or indium oxide (In₂O₃), instead of or in addition to magnesia.

In an example series of compositions (“B—Zn series”), which may be basedon the B1.5 sample composition discussed above, zinc ions may replacemagnesium ions. The B—Zn series may have a formula ofZr_(0.802+x)Sc_(0.198-2x)Zn_(x)O_(1.90). X values that may be used toprepare this B—Zn series of compositions are: 0, 0.0135 (correspondingto B1.5) and 0.027 (corresponding to B3), thereby creating the followingcompositions:

-   B1.5Zn: Zr_(0.815)Sc_(0.171)Zn_(0.0135)O_(1.90)-   B3Zn: Zr_(0.829)Sc_(0.144)Zn_(0.0270)O_(1.90)

Specifically, in this series, two Sc³⁺ ions may be replaced by one Zn²⁺and one Zr⁴⁺ ion. The oxygen stoichiometry remains constant as Sc³⁺ isreplaced by these ions. XRD patterns for sample compositions in thisB—Zn series of compositions are illustrated in FIGS. 11A-11C, whereFIGS. 11B and 11C are close ups of low and high angle regions of FIG.11A.

The conductivities at 850° C. of the B—Zn series compositions, comparedto corresponding B-series compositions and the parent composition with11 mol % Sc₂O₃, are provided in Table 3 below:

σ_(850° C.) (mS · cm⁻¹) B-Zn series B-series 11Sc₂O₃ 140 140 B1.5 127199, 232 B3 105 159

FIG. 12A illustrates D.C. conductivity at 850° C. versus ZnO content(measured as atomic percent of zinc) for the B—Zn series ofcompositions, overlaid onto a plot of D.C. conductivity at 850° C.versus magnesium content for B1.5 and B3.

In another example series of compositions, indium oxide may be used as aco-dopant with scandia. Similar to the A-series of compositions in whichMg²⁺ is a co-dopant that replaces Sc³⁺, in an In₂O₃ co-doped compositionthe In³⁺ ions may replace Sc³⁺ ions in a 1:1 ratio, with the oxygencontent remaining fixed. This series may be based on a parentcomposition of 11 mol % Sc₂O₃, and may have a formula of:Zr_(0.802)Sc_(0.198-x)In_(x)O_(1.90) where 0≤x≤0.198, such as0.018≤x≤0.18.

Sample compositions of this embodiment may have 0-11 mol % Sc₂O₃, suchas 1-9 mol % Sc₂O₃, and 0-11 mol % In₂O₃, such as 2-10 mol % In₂O₃, witha total doping range (i.e., sum of Sc₂O₃ and In₂O₃ mole percentages) of11 mol %. In one example, the composition may have 9 mol % Sc₂O₃ and 2mol % In₂O₃, and therefore have a formula ofZr_(0.802)Sc_(0.162)In_(0.036)O_(1.90).

FIG. 12B illustrates XRD patterns for this series, in which samplecompositions have discreet integer indium oxide contents varying from 0to 11 mol % and discreet integer scandia contents varying from 11 to 0mol %. FIG. 12C illustrates the variation in D.C. conductivity of thesesample compositions as a function of discrete integer indium oxidecontent (measured as mole percent of In₂O₃), where indium oxide contentvaries from 0 to 11 mol %. As shown by the data plot, D.C. conductivityof the samples may be a value between 80 mS/cm and 220 mS/cm. As alsoshown by the data plot, a peak D.C. conductivity level of at least 215mS/cm, such as between 215 and 220 mS/cm, may be achieved at about 3 mol% In₂O₃, which corresponds to a sample formula of aroundZr_(0.802)Sc_(0.144)In_(0.054)O_(1.90). Thus, this series may bedescribed as having a formula Zr_(1-w-y)Sc_(w)In_(y)O_(d), in which0.018≤w≤0.18, in which 0.018≤y≤0.18, and in which 1.8≤d≤2. In anembodiment, scandium ion concentration (w) may be characterized byw=0.198−y. In another embodiment, indium ion concentration (y) may becharacterized by y=0.054.

In another embodiment, scandia stabilized zirconia compositions that aresimilar to the H-series compositions may be doped with indium oxide(In₂O₃) in addition to magnesia. In an example series of compositions(“H—In series”), which may be based on the H2 sample compositiondiscussed above, Sc³⁺ ions may be replaced by Mg²⁺ ions in a 1:1 ratio,thereby lowering scandium and oxygen content of the composition whilekeeping zirconia and indium content constant. The H—In series may have aformula of Zr_(0.8)Sc_(0.15-x)In_(0.05)Mg_(x)O_(2-d), in which 0≤x≤0.05.The formula for the H—In series may also be written asZr_(0.8)Sc_(0.15-x)In_(0.05)Mg_(x)O_(d), in which 1.8≤d≤2 and 0≤x≤0.05.At x=0, no magnesia is present and a sample composition has a formulaZr_(0.8)Sc_(0.15)In_(0.05)O_(2-d). At x=0.05, a sample composition has aformula Zr_(0.8)Sc_(0.10)In_(0.05)Mg_(0.05)O_(2-d).

Another set of examples involves variants of the E1 sample composition,discussed above. In one example composition, zinc oxide may replacemagnesia as a dopant, producing a composition “E1-Zn” that may have aformula Zr_(0.815)Sc_(0.153)Y_(0.0153)Zn_(0.0135)O_(1.90). In anotherexample, In₂O₃ may be used as a co-dopant that replaces yttria,producing a composition “E1-In” that may have a formulaZr_(0.815)Sc_(0.153)In_(0.018)Mg_(0.0135)O_(1.90).

FIG. 13A illustrates energy-dispersive x-ray (EDX) spectroscopy analysisof the sample compositions E1-In and 9Sc₂O₃-2In₂O₃. FIG. 13B illustratesx-ray diffraction (XRD) patterns for these compositions, with anexpanded view region for angles of 2θ=80-88°. At 850° C., the D.C.conductivity of E1-In at 850° C. may be around 195 mS/cm, while the D.C.conductivity of 9Sc₂O₃-2In₂O₃ may be around 170 mS/cm. Thus, thesecompositions have a conductivity of at least 170, such as 180-195 mS/cm.

FIG. 14A shows bulk impedance values of the E1 sample composition withthe E1 variants E1-Zn and E1-In at 400° C. FIG. 14B shows bulkconductivity measurements for E1, E1-Zn and E1-In. FIG. 14C shows thetotal conductivity measurements for E1, E1-Zn and E1-In, and FIG. 14Dshows the conductivity across the grain boundary plane for E1, E1-Zn andE1-In.

In further embodiments, scandia stabilized zirconia may be co-doped withgallium oxide and yttria. Without wishing to be bound to a particulartheory, the combination of the smaller radius Ga³⁺ ion and the largerradius Y³⁺ may lead to less distortion of the crystal structure.

An example series of compositions (“I-series”) may be created with aformula Zr_(0.8018)Sc_(0.1782)Y_(0.02-x)Ga_(x)O_(1.90). In this seriesof compositions, one Ga³⁺ ion replaces one Y³⁺ ion, while zirconium,scandium and oxygen levels remain constant. The x values that may beused to form the I-series compositions are: 0, 0.005, 0.01, 0.015 and0.02, thereby creating the following compositions:

-   I0: Zr_(0.802)Sc_(0.178)Y_(0.02)O_(1.90)-   I1: Zr_(0.802)Sc_(0.171)Y_(0.015)Ga_(0.005)O_(1.90)-   I2: Zr_(0.802)Sc_(0.171)Y_(0.01)Ga_(0.01)O_(1.90)-   I3: Zr_(0.802)Sc_(0.171)Y_(0.005)Ga_(0.015)O_(1.90)-   I4: Zr_(0.802)Sc_(0.171)Ga_(0.02)O_(1.90)

At x=0, no yttrium ions are replaced, and no gallium ions are present.At the highest x value, x=0.02, all yttrium ions are replaced by galliumions.

Another example series of compositions (“J-series”) may be prepared witha formula Zr_(0.802)Sc_(0.188)Y_(0.01-x)Ga_(x)O_(1.90). In the J-series,one Ga³⁺ ion replaces one Y³⁺ ion, while zirconium, scandium and oxygenlevels remain constant. The x values that may be used to form theJ-series compositions are: 0, 0.0025, 0.005, 0.0075, thereby creatingthe following compositions:

-   J0: Zr_(0.802)Sc_(0.188)Y_(0.01)O_(1.90)-   J1: Zr_(0.802)Sc_(0.188)Y_(0.0075)Ga_(0.0025)O_(1.90)-   J2: Zr_(0.802)Sc_(0.188)Y_(0.005)Ga_(0.005)O_(1.90)-   J3: Zr_(0.802)Sc_(0.188)Y_(0.0025)Ga_(0.0075)O_(1.90)

At x=0, no yttrium ions are replaced, and no gallium ions are present.At the highest x value, x=0.0075, the composition contains 2.5 at. % Y³⁺and 7.5 at. % Ga³⁺ ions.

Another example series of compositions (“K-series”) may be prepared witha formula Zr_(0.8018)Sc_(0.1682)Y_(0.03-x)Ga_(x)O_(1.90). In this seriesof compositions, one Ga³⁺ ion replaces one Y³⁺ ion, while zirconium,scandium and oxygen levels remain constant. An x value that may be usedto form an example K-series composition is 0.015, which may create a K2sample composition having a formulaZr_(0.8018)Sc_(0.1682)Y_(0.015)Ga_(0.015)O_(1.90). At this x value, theatomic percentages of Y³⁺ ions and Ga³⁺ ions are equal.

Another example series of compositions (“L-series”) may be prepared witha formula Zr_(0.8018)Sc_(0.1582)Y_(0.04-x)Ga_(x)O_(1.90). In this seriesof compositions, one Ga³⁺ ion replaces one Y³⁺ ion, while zirconium,scandium and oxygen levels remain constant. An x value that may be usedto form an example L-series composition is 0.02, which may create a L2sample composition having a formulaZr_(0.8018)Sc_(0.1582)Y_(0.02)Ga_(0.02)O_(1.90). At this x value, theatomic percentages of Y³⁺ ions and Ga³⁺ ions are equal.

FIG. 15 is a ternary phase diagram showing the example I-, J-, K- andL-series of compositions. FIG. 16A illustrates XRD patterns for samplecompositions in these series, I2, J2, K2, and L2. FIG. 16B is anexpansion of the XRD patterns over the range of 2θ=82-85°. FIG. 17Aillustrates the variation in D.C conductivity with total gallium oxideand yttria content (measured as atomic percent of gallium or yttrium)for the I, J, K and L series of compositions at 850° C. FIG. 17Billustrates the variation in D.C. conductivity with scandium content(measured as atomic percent of scandium). As the data show, the K-seriescomposition, which has 0.015 at. % Y³⁺ ions and 0.015 at. % Ga³⁺ ions,has the highest conductivity, such as above 200 mS/cm, for example 221mS/cm.

D.C. conductivities of sample compositions I2, I3, J2, J3, K2 and L2 at850° C. are provided in Table 4 below:

Composition σ_(850° C.) I2 132 I3 126 J2 118 J3 129 K2 221 L3 106

Another example series of compositions (“M-series”) may be prepared witha formula Zr_(0.815)Sc_(0.15) Y_(0.02-x)Ga_(x)Mg_(0.015)O_(1.9). In thisseries of compositions, one Ga³⁺ ion replaces one Y³⁺ ion, whilezirconium, scandium, magnesium and oxygen levels remain constant. The xvalues that may be used to prepare the M-series are 0, 0.01 and 0.02,thereby creating the following compositions:

-   M0: Zr_(0.815)Sc_(0.15)Y_(0.02)Mg_(0.015)O_(1.9)-   M1: Zr_(0.815)Sc_(0.15)Y_(0.01)Ga_(0.01)Mg_(0.015)O_(1.9)-   M2: Zr_(0.815)Sc_(0.15)Ga_(0.02)Mg_(0.015)O_(1.9)    At x=0, no yttrium ions are replaced, and no gallium is present, and    at x=0.02, all yttrium ions are replaced with gallium ions.    D.C. conductivity results of the M-series compositions at 850° C.    are provided in Table 5 below:

Composition σ_(850° C.) M0 179 M1 154 M2 162

FIG. 18 is a graph illustrating the D.C. conductivity for the M-seriescompositions with varying gallium oxide content (measured as atomicpercent gallium).

Another example series of compositions (“N-series”) may be created witha formula Zr_(0.815)SC_(0.13)Y_(0.04-x)Ga_(x)Mg_(0.015)O_(1.90). In thisseries of compositions, one Ga³⁺ ion replaces one Y³⁺ ion, whilezirconium, scandium, magnesium and oxygen levels remain constant. The xvalues that may used to make the N-series are 0, 0.01, 0.02, 0.03, and0.04, thereby creating the following compositions:

-   N0: Zr_(0.815)Sc_(0.13)Y_(0.04)Mg_(0.015)O_(1.90)-   N1: Zr_(0.815)Sc_(0.13)Y_(0.03)Ga_(0.01)Mg_(0.015)O_(1.90)-   N2: Zr_(0.815)Sc_(0.13)Y_(0.02)Ga_(0.02)Mg_(0.015)O_(1.90)-   N3: Zr_(0.815)Sc_(0.13)Y_(0.01)Ga_(0.03)Mg_(0.015)O_(1.90)-   N4: Zr_(0.815)Sc_(0.13)Ga_(0.04)Mg_(0.015)O_(1.90)

At x=0, no yttrium ions are replaced, and no gallium is present, and atx=0.04, all yttrium ions are replaced with gallium ions.

In further embodiments, scandia stabilized zirconia may be co-doped withindium oxide and ceria. An example series of compositions (“O-series”)may be created with a formulaZr_(0.8o9)Sc_(0.182-x)Ce_(0.009)In_(x)O_(2-d), where 0≤x≤0.164. Thisseries may also be written asZr_(0.809)Sc_(0.182-x)Ce_(0.009)In_(x)O_(d), where 1.8≤d≤2 and0≤x≤0.164. In this series of compositions, In³⁺ ions replace Sc³⁺ ionsin a 1:1 ratio, while zirconium, cerium, and oxygen levels remainconstant. A parent dopant material for the O-series of compositions maybe 10Sc₂O₃-1CeO₂. Sample compositions may be created by replacing Sc₂O₃with up to 9 mol % In₂O₃ (e.g., 9Sc₂O₃-1In₂O₃-1CeO₂ to1Sc₂O₃-9In₂O₃-1CeO₂).

Other example compositions may be created with total dopant amountslower than 11 mol %. For example, one series of compositions may becreated with at least 8 mol %, such as 9 mol % of total dopant, and aformula Zr_(0.843)Sc_(0.0926-x)Ce_(0.009)In_(0.0556+x)O_(2-d), where0≤x≤0.0741. The formula for this series may also be written asZr_(0.843)Sc_(0.0926-x)Ce_(0.009)In_(0.0556+x)O_(d), where 1.8≤d≤2 and0≤x≤0.0741. In this series of compositions, In³⁺ ions replace Sc³⁺ ionsin a 1:1 ratio, while zirconium, cerium, and oxygen levels remainconstant. A parent dopant material may comprise 5Sc₂O₃-3In₂O₃-1CeO₂.Sample compositions may be created by replacing Sc₂O₃ with up to 4 mol %In₂O₃ (e.g., 5Sc₂O₃-3In₂O₃-1CeO₂ to 1Sc₂O₃-7In₂O₃-1CeO₂). In anotherexample, a series of compositions (“P-series”) may be created with 10mol % of total dopant, and a formulaZr_(0.825)Sc_(0.110-x)Ce_(0.009)In_(0.055+x)O_(2-d), where 0≤x≤0.0917.The formula for the P-series may also be written asZr_(0.825)Sc_(0.110-x)Ce_(0.009)In_(0.055+x)O_(d), where 1.8≤d≤2 and0≤x≤0.0917. In this series of compositions, In³⁺ ions replace Sc³⁺ ionsin a 1:1 ratio, while zirconium, cerium, and oxygen levels remainconstant. A parent dopant material may be 6Sc₂O₃-3In₂O₃-1CeO₂. Samplecompositions may be created by replacing Sc₂O₃ with up to 5 mol % In₂O₃(e.g., 6Sc₂O₃-3In₂O₃-1CeO₂ to 1Sc₂O₃-8In₂O₃-1CeO₂). Some x values thatmay used to make the P-series are 0 and 0.018, thereby creating thefollowing compositions:

-   P0: Zr_(0.825)Sc_(0.110)Ce_(0.009)In_(0.055)O_(1.92)-   P1: Zr_(0.825)Sc_(0.092)Ce_(0.009)In_(0.073)O_(1.92)

At x=0, no additional scandium ions are replaced, and the amount ofindium present is the same as in the parent dopant material.

Other embodiment compositions may be created with total dopant amountsthat are higher than 11 mol %, such as up to 14 mol %. For example, oneseries of compositions (“Q-series”) may be created with 11.5 mol % oftotal dopant, and a formulaZr_(0.801)Sc_(0.10-x)Ce_(0.009)In_(0.091+x)O_(2-d), where 0≤x≤0.082. Theformula for the Q-series may also be written asZr_(0.801)Sc_(0.10-x)Ce_(0.009)In_(0.091+x)O_(d), where 1.8≤d≤2 and0≤x≤0.082. In this series of compositions, In³⁺ ions replace Sc³⁺ ionsin a 1:1 ratio, while zirconium, cerium, and oxygen levels remainconstant. A parent dopant material may be 5.5Sc₂O₃-5In₂O₃-1CeO₂. Samplecompositions may be created by replacing Sc₂O₃ with up to 4.5 mol %In₂O₃ (e.g., 5.5Sc₂O₃-5In₂O₃-1CeO₂ to 1Sc₂O₃-9.5In₂O₃-1CeO₂). Some xvalues that may used to make the Q-series are 0 and 0.010, therebycreating the following compositions:

-   Q0: Zr_(0.801)Sc_(0.10)Ce_(0.009)In_(0.091)O_(1.91)-   Q1: Zr_(0.825)Sc_(0.091)Ce_(0.09)In_(0.10)O_(1.91)

At x=0, no additional scandium ions are replaced, and the amount ofindium present is the same as in the parent dopant material.

In another example, a series of compositions (“R-series”) may be createdwith 12 mol % of total dopant, and a formulaZr_(0.793)Sc_(0.110-x)Ce_(0.009)In_(0.090+x)O_(2-d), where 0≤x≤0.09. Theformula for the R-series may also be written asZr_(0.793)Sc_(0.110-x)Ce_(0.009)In_(0.090+x)O_(d), where 1.8≤d≤2 and0≤x≤0.09. In this series of compositions, In³⁺ ions replace Sc³⁺ ions ina 1:1 ratio, while zirconium, cerium, and oxygen levels remain constant.A parent dopant material may be 6Sc₂O₃-5In₂O₃-1CeO₂. Sample compositionsmay be created by replacing Sc₂O₃ with up to 5 mol % In₂O₃ (e.g.,6Sc₂O₃-5In₂O₃-1CeO₂ to 1Sc₂O₃-10In₂O₃-1CeO₂). Some x values that mayused to make the R-series are 0, 0.009, and 0.018, thereby creating thefollowing compositions:

-   R0: Zr_(0.793)Sc_(0.110)Ce_(0.009)In_(0.090)O_(1.90)-   R1: Zr_(0.793)Sc_(0.101)Ce_(0.009)In_(0.099)O_(1.90)-   R2: Zr_(0.793)Sc_(0.092)Ce_(0.009)In_(0.108)O_(1.90)

At x=0, no additional scandium ions are replaced, and the amount ofindium present is the same as in the parent dopant material.

In another embodiment, a related series of compositions (R′-series) alsohaving a total of 12 mol % dopant may be created with scandia stabilizedzirconia that is co-doped only with indium, and lacks cerium or ceria.The R′-series may have a formulaZr_(0.786)Sc_(0.143-x)In_(0.071+x)O_(2-d), where 0≤x≤0.125. The formulafor the R′-series may also be written asZr_(0.786)Sc_(0.143-x)In_(0.071+x)O_(d), where 1.8≤d≤2 and 0≤x≤0.125. Inthis series of compositions, In³⁺ ions replace Sc³⁺ ions in a 1:1 ratio,while zirconium and oxygen levels remain constant. A parent dopantmaterial may be 8Sc₂O₃-4In₂O₃. Sample compositions may be created byreplacing Sc₂O₃ with up to 7 mol % In₂O₃ (e.g., 8Sc₂O₃-4In₂O₃ to1Sc₂O₃-11In₂O₃). Some x values that may used to make the R-series are 0and 0.036, thereby creating the following compositions:

-   R′0: Zr_(0.786)Sc_(0.143)In_(0.071)O_(1.89)-   R′1: Zr_(0.786)Sc_(0.107)In_(0.107)O_(1.89)

At x=0, no additional scandium ions are replaced, and the amount ofindium present is the same as in the parent dopant material. At x=0.036,the amounts of indium and scandium are equal in the dopant material.

In another example, a series of compositions (“S-series”) having 13 mol% of total dopant may be created using scandia stabilized zirconiaco-doped with indium oxide and ceria. The S-series may have a formulaZr_(0.777)Sc_(0.107-x)Ce_(0.009)In_(0.107+x)O_(2-d), where 0≤x≤0.089.The formula for the S-series may also be written asZr_(0.777)Sc_(0.107-x)Ce_(0.009)In_(0.107+x)O_(d), where 1.8≤d≤2 and0≤x≤0.089. In this series of compositions, In³⁺ ions replace Sc³⁺ ionsin a 1:1 ratio, while zirconium, cerium, and oxygen levels remainconstant. A parent dopant material may be 6Sc₂O₃-6In₂O₃-1CeO₂. Samplecompositions may be created by replacing Sc₂O₃ with up to 5 mol % In₂O₃(e.g., 6Sc₂O₃-6In₂O₃-1CeO₂ to 1Sc₂O₃-11In₂O₃-1CeO₂). Some x values thatmay used to make the S-series are 0 and 0.018, thereby creating thefollowing compositions:

-   S0: Zr_(0.777)Sc_(0.107)Ce_(0.009)In_(0.090)O_(1.87)-   S1: Zr_(0.793)Sc_(0.101)Ce_(0.009)In_(0.099)O_(1.87)

In another embodiment, a related series of compositions (S′-series) alsohaving a total of 13 mol % dopant may be created with scandia stabilizedzirconia that is co-doped only with indium. The S′-series may have aformula Zr_(0.770)Sc_(0.142-x)In_(0.088+x)O_(2-d), where 0≤x≤0.125. Theformula for the S′-series may also be written asZr_(0.770)Sc_(0.142-x)In_(0.088+x)O_(d), where 1.8≤d≤2 and 0≤x≤0.125. Inthis series of compositions, In³⁺ ions replace Sc³⁺ ions in a 1:1 ratio,while zirconium and oxygen levels remain constant. A parent dopantmaterial may be 8Sc₂O₃-5In₂O₃. Sample compositions may be created byreplacing Sc₂O₃ with up to 7 mol % In₂O₃ (e.g., 8Sc₂O₃-5In₂O₃ to1Sc₂O₃-12In₂O₃).

FIGS. 19A-19C are graphs illustrating the variation in D.C. conductivityfor sample compositions of the O-series, P-series, Q-series, R-series,and S-series. FIG. 19A shows D.C. conductivity of sample compositions inthese series as a function of discrete integer scandium content(measured as at. % Sc³⁺), where scandium content varies from 7.2 to 18.2at. %. FIG. 19B shows the variation in D.C. conductivity of the samplecompositions as a function of indium content (measured as at. % In³⁺),where indium content varies from 0 to 10.9 at. %. FIG. 19C shows thevariation in D.C. conductivity of the sample compositions as a functionof the ratio of scandium content to indium content (excluding anO-series sample in which dopant composition had 0% indium oxide).

As shown by the data plots, D.C. conductivity of the samples may be avalue between 80 mS/cm and 220 mS/cm. As also shown by the data plot, apeak D.C. conductivity level of at least 215 mS/cm, such as between 215and 220 mS/cm, may be achieved in the O-series in a sample with around16.3 at. % scandium, and around 1.8 at % indium, which has a sampleformula of around Zr_(0.809)Sc_(0.163)Ce_(0.009)In_(0.018)O_(1.9). Theamounts of scandium and indium at the peak D.C. conductivity levelcorrespond to a dopant material of 9 mol % Sc₂O₃, 1 mol % In₂O₃, and 1mol % CeO₂. Thus, the O-series this series may be described as having aformula Zr_(1-w-y-z)Sc_(w)Ce_(z)In_(y)O_(d), in which 0.072≤w≤0.182, inwhich 0≤y≤0.1098, in which 0.008≤z≤0.1, and in which 1.8≤d≤2. In anembodiment, scandium ion concentration (w) may be characterized byw=0.182−y, and cerium ion concentration (z) may be characterized byz=0.009. In another embodiment, indium ion concentration (y) may becharacterized by y=0.018.

FIGS. 20A and 20B are graphs illustrating summaries of the D.C.conductivity results for various example series of scandia stabilizedzirconia compositions that may have the properties discussed above. FIG.20A shows D.C. conductivity of sample compositions in these series as afunction of discrete integer scandium content (measured as at. % Sc³⁺).FIG. 20B shows D.C. conductivity of the sample compositions in theseseries as a function of oxygen stoichiometry.

FIG. 21 is a ternary phase diagram showing sample high conductivitycompositions of embodiment B- and H-series scandia stabilized zirconiathat is doped with magnesia.

As demonstrated in the various example series of scandia stabilizedzirconia compositions, the B- and H-series compositions may have highrelative conductivities. For example, a sample B1.5 composition may havea D.C. conductivity of 232 mS/cm. In another example, sample H2 and H1compositions may have D.C. conductivities of 199 mS/cm and 171 mS/cm,respectively.

In another example, a sample K2 composition may have a relatively highconductivity of 145 mS/cm. In another example, a sample L2 compositionmay also have a relatively high conductivity of 145 mS/cm. In anotherexample, compositions doped In₂O₃ may have relatively highconductivities around 195 mS/cm.

The compositions above may be used for a solid oxide fuel cellelectrolyte. The electrolyte may be plate shaped with an anode electrodeon one side (e.g., a nickel and stabilized zirconia and/or doped ceriacermet) and a cathode electrode (e.g., lanthanum strontium manganate) onthe opposite side. The fuel cell comprising the electrolyte, anode andcathode electrodes may be located in a fuel cell stack. The term “fuelcell stack,” as used herein, means a plurality of stacked fuel cellsseparated by interconnects which may share common air and fuel inlet andexhaust passages, manifolds or risers. The “fuel cell stack,” as usedherein, includes a distinct electrical entity which contains two endplates which are connected to power conditioning equipment and the power(i.e., electricity) output of the stack. Thus, in some configurations,the electrical power output from such a distinct electrical entity maybe separately controlled from other stacks. The term “fuel cell stack”as used herein, also includes a part of the distinct electrical entity.For example, the stacks may share the same end plates. In this case, thestacks jointly comprise a distinct electrical entity, such as a column.In this case, the electrical power output from both stacks cannot beseparately controlled.

The formulas that represent the compositions above are not intended tolimit the scope of the invention to particular atomic or molepercentages, but are provided to facilitate disclosure of the variousseries of related compositions. For example, the representation ofoxygen as “O_(2-d)” or “O_(d)” provides for a variable amount of oxygenthat may depend, for example, on the total amount of doping, valence ofcations in the composition, etc. Example amounts of oxygen that may bepresent in series of compositions discussed above include, withoutlimitation: 1.92 at % oxygen in the P-series; 1.91 at % oxygen in theQ-series, 1.90 at % oxygen in the R-series; 1.89 at % oxygen in theR′-series; and 1.87 at % oxygen in the S-series.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

The invention claimed is:
 1. An electrolyte composition for a solidoxide fuel cell, having a formula Zr_(1-w-x-z)Sc_(w)Mg_(x)Y_(z)O_(d),wherein: 0.1≤w≤0.18, 0<x≤0.03, 0<z≤0.08, and 1.8≤d≤2.
 2. The electrolytecomposition of claim 1, wherein w=0.171−z, wherein 0.013≤x≤0.014.
 3. Theelectrolyte composition of claim 2, wherein z=0.018.
 4. The electrolytecomposition of claim 1, wherein w=0.144−z, and wherein 0.02≤x≤0.03. 5.The electrolyte composition of claim 1, wherein 0.018≤z≤0.072.